The Kenya police force has listed seven buildings, including the headquarters of the revenue authority, as being under immediate risk of a terror attack from an al-Qaeda-linked militant group that has threatened to bring down skyscrapers in Kenya’s capital city, an internal UN security report says.

Somalia’s al-Shabaab militant group last week said it would attack Kenya’s skyscrapers within two weeks, a warning that followed a bomb attack in Nairobi’s city centre which killed one person and injured 32 others. Al-Shabaab’s warning was posted by the private intelligence firm, IntelCenter.

It was the third time al-Shabaab has threatened to bomb Kenya after the country sent its troops into Somalia to pursue the militants in October.

The UN report circulated late on Wednesday says police are restricting parking at the buildings listed and are screening all people going into them. The other buildings named in report are Kenyatta International Conference Centre, I&M Building, National Social Security Fund Building, Lornho House, View Park Towers and Nyayo House.

Police spokesman Eric Kiraithe said the UN report is misleading and denied designating any building as being under threat. Kiraithe said Nyayo house is a heavily guarded government building which houses the Ministry of State for Immigration and Registration Persons and does not need additional security.

The Virginia-based IntelCenter said last week that the towers most likely to be targeted are those housing hotels, those frequented by Westerners, government offices, media and prominent corporations.

"The building volume of threats and low-level activity indicate that such an attack may be attempted sooner rather than later," IntelCenter said.

Some of the buildings named in the UN report fit this profile.

Test run

The report says the latest threat by al-Shabaab is considered serious because of last week’s bomb blast which was different from grenade attacks blamed on Kenyan sympathizers of al-Shabaab. Since October, the grenade attacks have killed at least 40 civilians.

Police concluded that an improvised explosive device caused last week’s blast that ripped through a building full of small shops in downtown Nairobi. The explosion sent dark smoke billowing out of a one-story building on the avenue named after Kenya’s second president, peeled back the front corner of the aluminium roof and shattered windows in the building.

A high-rise building with a glass exterior next door was largely untouched. The UN said it could have been a test run for a bigger attack.

"In light of the device which was used last week (improvised explosive device designed for structural and not fragmental damage) in what appeared to be a familiarisation exercise, the threat is considered very high," the report UN report said.

Al-Shabaab is waging an insurgency against a weak UN-backed transitional government in Somalia. It carried out its first international attack in Uganda, killing 76 people watching the World Cup final on TV in July 2010.

In April the US government warned that it continues to receive information about potential terrorist threats aimed at US, Western and Kenyan targets inside Kenya.

The Obama administration was expected to announce on Thursday that it will offer up to $33m in rewards for information about top members of al-Shabaab.

It will offer up to $7m for al-Shabaab’s founder, Ahmed Abdi aw-Mohamed; up to $5m each for his associates, Ibrahim Haji Jama, Fuad Mohamed Khalaf, Bashir Mohamed Mahamoud and Mukhtar Robow; and up to $3m for Zakariya Ismail Ahmed Hersi and Abdullahi Yare.